Bacterial vaginosis is a syndrome manifested by adverse alterations in vaginal microbial flora. It is present in 10 to 25% of women in the general population, and is the most common vaginal infection. It has been associated with a number of adverse sequelae in reproductive health including acquisition of HIV, pelvic inflammatory disease, cervical dysplasia, and complications of gynecological procedures. During pregnancy it has been associated with an increased risk of preterm birth and postpartum maternal infection. However, the vaginal microbial flora remains poorly understood. Information on variation in microbial flora and its association with BV from a longitudinal study is unavailable. This prospective observational study aims to advance our knowledge of the natural history and abnormal alteration in vaginal microbial flora, and to identify risk factors that are associated with these changes. A total of 5500 healthy, non-pregnant women of reproductive age will be enrolled and followed up in Birmingham, Alabama, for one year (baseline visit plus once every three months afterwards, totaling 5 visits). Detailed information on sociodemographic status, medical history, hygiene practice, sexual behavior and psychosocial stress will be collected through in-person interviews. Routine gynecological and dental examinations will be carried out during the study visits. Samples of vaginal secretion and blood will be collected. Laboratory tests to quantify vaginal microbial flora and to identify infection of bacterial vaginosis as well as common sexually transmitted diseases will be conducted.